Phospholamban (PLB) is a protein which inhibits the SR Ca2+-ATPase. Phosphorylation of PLB relives this inhibition in vitro and there is substantial evidence that PLB modulates cardiac contractility in vivo. There is some evidence for a similar role for PLB in smooth muscle. Recently, Dr. Evangelia Kranias has produced a PLB "knockout" (PLB-) mouse. The investigator proposes to use this mouse as a model to study the role of PLB in smooth muscle in vivo. Preliminary data are presented to support the hypothesis that PLB is an important modulator of intercellular Ca2+ and a significant determinant of smooth muscle contractility. This project has three specific aims. In the first aim, the investigator proposes to quantitate and compare contractility in smooth muscle from the PLB- mouse to that in age-matched wild type mice. The initial studies will focus on the tonic aorta and the phasic portal vein as representative vascular preparations and the longitudinal ileum and trachea preparations, representative of enteric and airway smooth muscle. These studies will also be extended to resistance arteries. The role of PLB in baseline contractile functions, as well as its importance in relaxation mediated by CAMP or CGMP pathways will be determined. In this Aim the investigator will also verify the presence of PLB in wild type muscles and establish the levels of this protein and of the Ca2+- ATPase. The second Aim will test the hypothesis that alterations in contractility in PLB-smooth muscles are due to changes in Ca2+ handling attributable to altered SR function. These tests will include fluorescent measurements of (Ca2+)I and the use of specific inhibitors of SR Ca2+ release and Ca2+ uptake. A goal of this aim is to determine the magnitude and investigate the nature of potential mechanisms that may compensate for the absence of PLB. The third aim is to determine the effects of the absence of PLB on the Ca2+ pump rate in vivo, and to quantify the baseline level of PLB inhibition of the pump in normal tissue.